A method of this type is already known from DE 41 16 518 C2.
The work WKA of the gas force transmitted by the cylinder pressure during a work cycle to the piston is determined as the ring integral of the cylinder pressure p over the (stroke) volume change dVα, which is a function of the piston path (i.e., of the crankshaft angle α), compare, for example, “Handbuch Verbrennungsmotor [Handbook of Internal Combustion Engines]”, by Richard van Basshuysen/Fred Schäfer, second edition, June 2002, chapter 3.6. The variable WKA may be determined by a simulation model or from the p-V diagram by planimetration (measuring the area content). However, the typical method also used in the patent specification according to the species comprises sampling the cylinder pressure discretely via the angle encoder (α) and calculating WK (at least in a predetermined KW range) via a numeric integration as a summation function. Calculating the angle-dependent volume change over the engine geometry, storing it in a table, and then outputting it in accordance with the current crankshaft angle and finally using it in the calculation of WK are also known for simplifying the calculation.
It is known that criteria, which are directly relevant for the regulation and control of the engine, may be derived from the cylinder pressure curve of an internal combustion engine. The internal or indexed mean pressure pmi is frequently selected as a significant variable for describing the gas work performed during the engine combustion. It is an equivalent to the specific work acting on the piston and is determined as the integral from the cylinder pressure curve over the stroke volume Vh of a work cycle (p-V diagram). Variables such as power and torque may also be calculated therefrom.
Because no cylinder pressure sensors have been used up to this point in internal combustion engines in large-batch manufacturing, the ascertainment of engine variables of this type has been restricted to specially prepared experimental engines, which have predominantly been used for development and research purposes. However, it is to be assumed that in the course of continuously stricter exhaust regulations that cylinder pressure sensors will also be used in the future in mass-produced engines, because they deliver important information about the combustion process. They represent a valuable instrument for the process monitoring and control and thus for maintaining the applicable emission limiting values. This relates to both gasoline and also diesel engines. The gasoline-engine CAI method (controlled auto ignition), which is currently being developed for start of production, is especially noted in this context. Because of the sensitivity of this combustion method, the use of cylinder pressure sensors is indispensable therein for the combustion process control and/or regulation.
The calculation of specific engine variables such as the internal mean pressure pmi or the combustion focal point has typically been performed until now, as described above, using step-by-step integration at relatively high sampling rates (e.g., in 1° KW steps) and/or resolution, so that a high-performance measurement and data processing technology is required. A conventional engine control unit (ECU) will typically not suffice for this power and performance demand, however.
Moreover, a method for converting highly dynamic sensor signals for an engine controller is also known from DE 101 27 913 B4. Because systematic errors may occur in the digital sampling of the sensor signals and/or engine state variables, which are each detected at individual points in time, it is suggested that the signal of a cylinder pressure be integrated using an integrator in particular for a predefined short time span in comparison to the work cycle and the resulting integrator value be input into the engine controller, the time span being selected in such a way that the integrator value permits a conclusion about the overall signal curve of the cylinder pressure because of better digital resolution capability.